Reaction dynamics of the [formula omitted] system: chemiluminescence, energy disposal and product polarization

Abstract

The Ca( 1 D 2, 3 P J) + CH 3 → CaI(A,B) + CH 3 reactions system has been studied by measuring its chemiluminescence under beam-gas conditions. Absolute values of the state-to-state reaction cross-sections were determined at low collision energy E T ⩽ 0.20 eV . In addition, the electronic branching ratio and product energy disposal have been determined for each metastable reaction. The major changed observed in the chemiluminescence when comparing the Ca( 1D 2) reaction versus that of Ca( 3 P J) is the total yield associated with the former reaction. To the best of our spectral resolution neither the electronic branching ratio e.g. CaI(A)/CaI(B) nor the internal CaI energy disposal change significantly as the metastable Ca( 1 D 2)/ Ca( 3 P J) ratio is varied. In spite of the fact that the Ca( 3 P J) reaction is less exoergic, the CaI product appears with a higher fraction of internal energy than that of Ca( 1D 2) reaction. Thus, the fraction of the total energy appearing in CaI internal energy amounts to 57.5% in the Ca( 3 P J) reaction while it is 19.3% only for the Ca( 1D 2) reaction. This difference is discussed in the light of a distinct mechanism associated with the attack of the excited Ca atom into the CI bond. No significant chemiluminescence yield was found for the energetically open CaCH ∗ 3 channels. The product chemiluminescence polarization was also measured as a function of the metastable concentration. A significant degree of polarization was found depending upon the specific electronic excitation. The analysis of the polarization emission associated to the parallel CaI(X 2Σ + ← B 2Σ +) emission led into a strong polarization of the product rotational angular momentum. The comparison of the product rotational alignment for the kinematically identical Ca( 1 D 2, 3 P J, 1 P 1) + CH 3 → CaI ∗ ( B 2Σ +) + CH 3 reaction system showed that the CaI rotational polarization diminishes in the 3 P J → 1 D 2 → 1 P 1 sequence, e.g. as the reaction exothermicity increases. In addition the degree of polarization associated with other emission bands as for example CaI(X 2Σ + ← A 2Π 1 2 ) indicates the presence of a parallel transition which was been interpreted as mixing of Hund's case (a) and (c) appropriate for this heavy CaI diatom produced with a high rotational excitation.